1. Field of the Invention
The present invention relates to a spreading and lap-forming machine for converting a web to a lap by folding in pleats.
2. Description of the Prior Art
In a machine of this type as disclosed, for example, in patent document FR-B-2,553,102, a web delivered by a card is first conveyed by a first conveyor-belt or so-called front belt to a pinching zone in which it is maintained between the first belt and a second belt and conveyed to an output carriage. The reciprocating motion of the output carriage above a receiving apron causes deposition of the web which said carriage discharges onto the apron in alternate accordion pleats. Since the apron moves in a direction parallel to the axes of the guide rollers carried by the output carriage, the successive pleats are relatively displaced along this direction and the web thus forms on the apron a continuous lap, the lateral edges of which are defined by the pleats. The beginning of the pinching zone is defined as a position in motion by an input carriage which performs a reciprocating movement. This movement is intended to ensure that the speed of admission of the web into the spreading machine is made compatible with the speed of unwinding of the web by the output carriage in spite of the reciprocating movements of said carriage. Thus the web, which has a fragile structure, is not disturbed either by packing or by stretching or by friction.
The developed length of the closed paths followed by the conveyor-belts must be maintained constant in spite of the reciprocating movements of the carriages.
In some designs, the reciprocating motion of the input carriage is compensated by the contrary reciprocating motion of the output carriage and conversely by means of a suitable arrangement of the guide rollers (reference can be made, for example, to patent document FR-B-2,553,102).
In other designs (see FR-B-2,234,395 or US-A-1,886,919, provision is made for auxiliary carriages which also carry out reciprocating movements of translation in order to selectively lengthen or shorten the conveyor-belts outside the pinching zone and thus to compensate for variations in length of the pinching zone.
The present Application is more particularly concerned with this second type of construction.
The main carriages and auxiliary carriages are usually driven by means of motion-transmission elements such as toothed drive-belts or chains disposed in meshing engagement with driving pinions, the rotation of which is controlled. One end of each motion-transmission element is attached to a main carriage and the other end is attached to an auxiliary carriage, the motion of which has to be synchronized with that of the main carriage in order to maintain constant the length of the closed path followed by one of the conveyor-belts. The rotation of the pinion in a first direction pulls the main carriage over one-half of its reciprocating motion. All other things being equal, this would tend to increase the length of the corresponding closed path. The conveyor-belt which follows this closed path is therefore subjected to a tractive force which is transmitted to the auxiliary carriage and thus initiates the movement of translation of said carriage which is necessary in order to maintain the length of the closed path at a constant value. Similarly, the rotation of the Dinion in the second direction pulls the auxiliary carriage in the other direction, thus applying a tractive force to the conveyor-belt, this tractive force being transmitted to the main carriage in order to cause this carriage to carry out the second half of its reciprocating motion.
The mode of driving described in the foregoing is often preferred since it advantageously reduces the number of control elements, a limited number of driving pinions being sufficient to carry out the movements of translation of four carriages in each direction.
Nevertheless, this construction has a disadvantage in that the conveyor-belts are continuously put under tension as a result of the inertia of the carriages. Since the conveyor-belts follow sinuous paths at relatively high speed, repeated tensionning of the belts causes a certain degree of wear. This usually produces a fissured appearance of the belt surface at the expense of uniform and accurate transfer of the fibers constituting the web.
The conveyor-belts are therefore replaced periodically, which is costly and entails the need to withdraw the machine from service over a fairly long period of time. In order to offer resistance to wear, the conveyor-belts can be of relatively substantial thickness but in that case move along their sinuous paths with greater difficulty.
One of the objects of the present invention is to solve the problems mentioned above by proposing a novel spreading and lap-forming machine in which the conveyor-belts have enhanced durability and/or a smaller thickness.